The present invention relates generally to an improved current conductor or drain wire for a metal oxide electrode in an alkaline electrolyte system, particularly one which is provided with an additional foreign metal to reduce contact resistance.
The present invention generally relates to primary galvanic cells including a pressed metal oxide positive electrode, a negative electrode prepared from zinc powder, and an alkaline electrolyte taken up in a separator; disposed in a steel casing. The steel casing generally serves as the current conductor for the positive active mass (e.g., manganese dioxide or silver oxide).
Experience has shown that such primary cells are subject to the disadvantage that their discharge rate after prolonged storage is far lower than in their freshly prepared state. This is because during storage and under certain conditions, especially in cells of the alkaline MnO.sub.2 /Zn primary system, a poorly conducting cover layer is formed on the current conductor of the positive metal oxide electrode which leads to an undersirable increase in the internal resistance of the cell. Since the manganese dioxide is pressed directly to the casing cup (made of steel or nickel-plated steel), the reduced service readiness of stored cells is explained by oxidation of the nickel layer, and the consequent increase in contact resistance in the transition between the metal oxide and the positive current conductor. The formation of such poorly conducting interfaces during the contact of a current conductor with an electrochemically active positive substance also occurs in primary cells with solid electrolytes.
As can be determined from U.S. Pat. No. 2,861,116, one attempt to remedy this situation was to use a material for the current conductor which is as inert as is possible. Platinum, palladium, tantalum, molybdenum, silver, nickel, lead, gold, titanium, zirconium and carbon were found to be suitable in this regard, among others. However, in German Patent No. 1,421,582, such teachings are dismissed as being too general, and gold is recommended as the preferable agent of choice. This takes the form of a gold coating on the positive steel container for the described galvanic primary cell (which is among the cells encompassed by the present invention).
A rather significant disadvantage to gold or gold-plated current conductors is the cost resulting from the selected material, and the processes which must be employed in using such material. Moreover, if mercury oxide is used as the positive electrode material, the gold will gradually become amalgamated during discharge of the mercury oxide. Thus, the utility of such a system is called into question due to the instability of the initially low contact resistance during storage. This problem is even more pronounced when silver is used as the current conductor material, since silver is also readily amalgamated by HgO and is eventually dissolved during operation of the cell.